<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function isRunyear(year) {
        if ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0) {
          return true;
        } else {
          return false;
        }
      }
      //   var res = isRunyear(2000);
      //   console.log(res);

      function alertDay(year) {
        var res = isRunyear(year);
        if (res) {
          alert("2月有29天");
        } else {
          alert("2月有28天");
        }
      }

      alertDay(1996);
    </script>
  </body>
</html>
